The present invention relates to a method and device for regulating the electrical bias voltage at the measuring capacitor of a MEMS sensor element, in which a base voltage is applied to the measuring capacitor, and in which this base voltage is subsequently regulated in such a way that the potential difference between the two electrode sides of the measuring capacitor corresponds to the setpoint voltage.
The regulation of the electrical bias voltage at the measuring or microphone capacitor of MEMS microphones is of particular significance. These generally include a sound pressure-sensitive diaphragm and a fixed counter-element. The diaphragm and the counter-element act as supports for the flat electrodes of the microphone capacitor, so that the changes of the distance between the diaphragm and the counter-element caused by sound pressure are detectable as capacitance fluctuations of the microphone capacitor. To increase the sensitivity of such MEMS microphones, a mechanical preload is applied to the diaphragm by applying a direct voltage to the microphone capacitor. This draws the diaphragm toward the counter-element electrostatically, the electrostatic force of the direct voltage counteracting the spring force of the diaphragm. This direct voltage may only be increased to the so-called pull-in point, at which the electrostatic force is equal to the spring force of the diaphragm. If the pull-in voltage is exceeded, the diaphragm snaps abruptly against the counter-element, as a result of which the microphone capacitor is short-circuited. Since the diaphragm at the pull-in point is in equilibrium of forces, each external force effect results in a diaphragm deflection, which is counteracted by no or only a very slight spring force. Consequently, the sensitivity of the diaphragm is highest at the pull-in point. If a MEMS microphone is to be operated in the range of maximum sensitivity, the electrical bias voltage at the microphone capacitor must be continuously monitored and regulated to the pull-in voltage. The pull-in voltage of MEMS microphones typically lies in the range of 5 V through 8 V. For regulating the electrical bias voltage of MEMS microphones, regulators are therefore used in practice, the output stage of which is able to regulate voltages of this magnitude.